1. Field of the Invention
The present invention is generally directed toward a method for improving cornering performance in a four wheel drive vehicle by scaling the side-to-side drive torque bias in response to changes in total driveline torque.
2. Description of Related Art
In the area of electronic drive torque distribution control, it is possible to shift drive torque to various driven wheels in accordance with specific vehicle operational conditions and sensed driver needs. During cornering it is advantageous to send more drive torque to the outside wheel of a given drive axle to enhance the transient characteristics of the vehicle's turning behavior.
During situations of combined acceleration and turning it can be particularly difficult to achieve consistent line trace performance when the drive torque is increased and/or decreased either by driver demand and or variations in system functioning. In an attempt to maintain consistent line trace while the drive torque is varied, it is important to maintain a relatively constant yawing moment from the distributed drive torque.
In various state of the art driveline control systems, there is a consistently varying yaw moment from the drive torque at a given trimmed lateral acceleration. As the drive torque increases and/or decreases, the amount of yawing moment changes and the overall line trace is not maintained.
Current practice is illustrated in FIG. 1. The drive torque distribution on a given axle (side-to-side distribution) is biased toward the outside wheel (as represented by different arrow sizes on the wheels) in order to provide enhanced traction and line trace capability. This side-to-side bias (or torque split) creates an “inward” turning moment on the vehicle body and helps to direct the vehicle inward in the turn as drive torque increases. As the total drive torque increases and/or decreases, and the side-to-side bias ratio does not change, the amount of inward yawing moment changes according to the delivered level of drive torque. Because of this behavior, perturbations in the line trace behavior occur. Starting from a quasi-equilibrium position at a prescribed lateral acceleration, as total drive torque is increased the inward yawing moment increases and the vehicle begins to “tighten its line”. Conversely, starting from a quasi-equilibrium position at a prescribed lateral acceleration where there is an existing high level of drive torque and drive torque is lessened, the vehicle begins to “loosen its line” as the inward turning moment is released. Both situations cause noticeable and undesirable disturbances in the driving trajectory. A method and system that avoids these situations is desired.